The present invention relates to a wiring board which exhibits a very small delay time for use in large sized to middle or small sized computers and flexible printed boards which are essential for miniaturization of electronic parts.
Hitherto, there have been used multi-layer wiring boards comprising an epoxy or maleimide resin-glass cloth laminate and copper foil or an alumina/tungsten multi-layer wiring boards. However, in the former case, there are limitations in production method and materials and fine wire width of less than 30 .mu.m cannot be obtained and in the latter case, tungsten of high heat resistance with high resistivity must be used because ceramics are high in dielectric constant and a step of very high temperature is required for production. Thus, wiring boards of high performance have been demanded as future high speed and high density wiring boards to be used in place of the above conventional wiring boards. The first possible one is a copper/polyimide wiring board. This is produced by forming polyimide of low dielectric constant and copper of low resistivity on a silicon wafer or ceramic substrate by the same fining process as used for production of semiconductor wiring. By this method, it can be expected that wiring board of high performance is obtained. As such conventional technique, there is a polyimide laminate material disclosed in Japanese Patent Kokai No. 57-181857.
However, there is the problem that when polyamic acid which is a polyimide precursor is heated and cured in contact with copper or silver at higher than 300.degree. C., a heat decomposition reaction occur which cannot be considered to occur in view of the heat resistance of polyimide per se. For example, it has been known that when polyamic acid varnish is coated on a film of copper and heat cured, distinct discoloration is recognized at higher than 300.degree. C. and the film becomes mechanically very brittle. Similar phenomena are recognized to occur at production of flexible printed board or curing of electrically conductive silver paste. This problem is especially conspicuous for copper and silver and nearly no problem is seen in aluminum, titanium, nickel and chromium.
Up to now, in forming an imidized film in contact with a metal, the following methods have been employed, namely, a method of forming a metallic film such as of inert chromium or an inert film of SiO.sub.2 or Si.sub.3 N.sub.4 and coating thereon a polyamic acid precursor varnish and heating and curing the varnish and a method of heating and curing the varnish in a reducing atmosphere such as hydrogen when the varnish is allowed to directly contact with the metal. These methods suffer from severe problems such as much increase in the number of steps and increase in running cost.
Taking the hint from the fact that even if a film of such metal is formed on a polyimide film and heater, substantially no adverse effect is given to the polyimide film, the inventors coated a varnish of polyimide on a metallic film and heated and cured the varnish and as a result have found that no deterioration of polyimide occurs. However, this method still has the problems that since solubility of polyimide is very poor, cresol type solvents which are harmful to human bodies must be used and only special solvents can be used.
The inventors have carefully studied chemical reaction of polyimide precursor and metal which takes place in production of a metal/polyimide composite which includes a step of thermal imidization of polyimide precursor in contact with the metal. As a result, it has been elucidated that the metal is dissolved in the presence of carboxylic acid groups of polyamic acid and subsequently when it is exposed to high temperature, imide ring is decomposed by the metal and simultaneously particulate metal oxide is precipitated in the polyimide film. Furthermore, it has also been found that similar phenomenon is also recognized for polyimide precursor having sulfonic acid group and presence of acidic functional group causes dissolution of metal.